Initiating in situ combustion in a carbonaceous stratum



Oct. 17, 1961 J. w. MARX ETAL INITIATING IN SITU COMBUSTION IN ACARBONACEOUS STRATUM Filed May 19, 1958 PRODUCTION AIR AIR

INVENTORS J.W. MARX J.C."TRANTHAM H.W. PARKER A TTORNEVS nited StatesPatent .IN SITU COMBUSTION IN l CRBGNCEOUS John We Marx, Joseph C.Trairtham', and Harry W. Parker, Bartlesville, Okla., assign'ors toPhillips Petroleumompfalry; .a Corporation Vof Delaware Filed May/1.9,1958, Ser.,No. 736,340 11 Claims. (Cl. 166-11) This invention relates toa process for igniting and establishing" in situ 'combustion in apermeable carbonace'ous stratum. p

In situ combustion inA the recovery of hydrocarbons from' undergroundstrata containing carbonaceous ceous material is becoming `moreprevalent in the petroleurn4 industry. In this" technique of production,combustion is initiated in the carbonaceous stratum and the resultingcombustion zone caused to move through the stratum by either inverse ordirect air drive whereby the heat of combustion of a substantialproportion of the hydrocarbon in the stratum drives out and, in the'case of inverse driye, upgrades a substantial pnoportion of the unburnedhydrocarbon material. p

The ignition of carbonaceous material inV a stratum around a boreholetherein, followed by injectionl of air through the ignition borehole inthe stratum, constitutes a directl airy drive process for effecting insitu combustion and recovery'l ofhydrocarbons from the stratum. In this'type of operation the stratum frequently plugs in front of thecombustion zonebecause a heavy viscous liquid barili of hydrocarboncollects in the stratum in ad- Vance of the combustion zone whiohprevents movement of air tothe combustion proces. To overcome thisdifeulty and to permit the continued progress lof the cornbustion zone'.throughthe stratum, inverse air injection has been resorted to. B`y`this technique, a combustion zone is established arouidfan ignitionborehole by any suitableY means and air `is` fed through the stratum tothe combustion zone from one or more surrounding boreholes.

In orderto initiate in situ" combustion! in a` carbonaceous stratum,such asan oil bearing sand or a porous coal vein or anypetroliferouslstructure which is naturally porous orrendered permeableby fracturing, it is necesto apply heat tothe stratum until itstemperature reaches or errceeds the ignition temperature of thecombustible material in the' stratum and then contact the hot stratumVwith O2. The' heating may be accomplished in any of a number ofvVayss'uch as by a gas fireddovvnhole burner, an electric heater, o1"Y`a mass of burning material positioned the borehole. In theseconventional methods of' heating the stratum around a borehole air orIother oxygeny containing gasis injected thrul the borehole into thestratum so as to carry heat thereto and bring the same to the requiredVtemperaturefor ignition. Inusing these methods the stratum adjacent theborehole is contaminated with combustion gases and ashes from thecombustible materia1,when such is utilized. Another disadvantage lies inthe overheating and fusingA of the siliceous structure" of 'the stratumso as to impair'the permeability thereof; In some cases the overheatingcauses' spallng and caving in the borehole Withinr the straturn.

This invention is concerned' with a method of ignit'i'ng andestablishingiii situ combustion in a carbonaceosstratum while avoiding'the disadvantages enumerated above. i 4 p i `It is accordingly, anobject ,of the invention toprovide an improved process for" igiting acarbonaceous stratum around iii`flillctlle'n.` Anhei Object ist() aVidoverheating of av carbonaceous stratum during initiation of combustiontherein. A further' object is to' provide a processt'for igniting acarbonaceous stratum utilizing burning charcoal which avoids blowing thecharcoal out of the borehole. A- still further object is to providey aprocess for ignitfing a stratum without contaminating the stratum witheither ashes or combustion gases. Another objective ofthe process is toprovide control of the burning rate of the charcoal. An additionalobject is to prevent soaking of the charcoal with formation water andmaintain the charcoal in combustible condition. Other objects of theinvention will become apparent upon consideration of the accompanyingdisclosure.

A broad aspect of the invention comprises burning a porous combustiblemass' of fuel in a borehole in a carbonaceous stratum utilizing a supplyof oxygen for the burning jgr'ocess passing into the borehole from thestratum. The usual source of oxygen is air, but, Where the term air isutilized in this specification, it is to be understood thatoxygen-enriched air, diluted air, or pure oxygen may be utilized, aslong as the O2 concentration in the gas is' combustion supporting.

The combustible fuel is ignited by any suitable means and' air is passedthru the'stratum tothe ignition borehole from at least one andpreferably a plurality of air injection boreholes surrounding theignition borehole and spaced therefrom a relatively short distance`which permits injection of air into the ignition borehole withoutexcessive injection borehole pressures. Usually a spacing in therang'ejof several feet to 100 feet is feasible, although the injection"boi-'elzlolesV may be closer or farther from the ignition'borehole insome strata.

The' preferredv fuel comprisesV charcoal briquettes packed into theignition borehole from the bottom level of the stratum to the topthereof. Charcoal in other forms may also be utilized so long as thecharcoal mass is gas permeable; One desirable arrangement comprises aperforate section of :tubing Within the stratum connecting with aniinperforate tubing string above the stratum, with an annula'r ma's's ofcharcoal briquettes or other particulate charcoal packed aroundl theperorate tubing. The perforations in the tubing should, of course,bes'maller than tli'par'ticulatecha'r'coal s`o` as to avoid blowing thecharcoal out: thr'u the tubing' when combustion'supporting gas isinjected into the borehole from the surrounding stratum and combustiongas is vented up the borehole.

Another desirable arrangement comprises agas'` pervious mass'of'char'coalbnded together into a cylindrical form, slightly smallerthan the borehole and casings thru which it is to be introduced to thestratum, which is supported oni the lower end of a rod or' tubingstring. In case' a tubing string is utilized to support the preformedcharcoal cylinder the tubing Within the charcoal may be perforate so asto permit recovery of thec'oinbustion gases thru the tubing stringdirectly from the charcoal. In this embodiment of the invention thelower end of the charcoal mass-is' ignitedV by any suitable means suchas' an electric heating coil, powered from the surface, or by bottomhole batteries triggered by jarring, a vial of sodium peroxide embeddedin the charcoal so that it breaks upon contact with the well bottom and.ignites' the charcoal, a thermite capsule, a fuse,l etc. j

Once the lower tip ofthe combustible rod or cylinder is burning, thewhole assembly, including the rod and support means, may be raised orlowered so as to progressively ignite the stratum along any selectedsection thereof from top'to bottom4 or vice versa. As the r'od orcylinder burns away, additional rod is fed downhole. In utilizinginver's'eair injection thru the stratum, formation-produced air (airforced thru the stratum into the ignition borehole) is more thanadequate to sustain the combustion, the excess being sufcient toinitiateV combustion withinlth'e stratum onceprthe temperature has beenraised to the ignition level; in this type of operation the resultingash `is blownV out the production line with the exhaust gas and does notcontaminate the stratum. When direct injection of air thru the ignitionborehole is utilized, the ash is deposited at the bottom of the boreholeor on the face of the stratum. In either case, the glowing end of therod can be made to traverse the pay thickness at any desired rate,independently of the normal attrition rate of the rod, until thecombustible stratum is ignited from top to bottom thereof within thewalls of the borehole. Position of the burning end of the rod may bedetermined by sliding which FIGURE l is an elevation partially insection showing an arrangement of boreholes and apparatus therein for'effecting one embodiment of the invention; FIGURE 2 is an elevation inpartial section of apparatus in an ignition borehole in accordance witha second embodiment of the invention; and FIGURE 3 is a transversesection of the fuel element in FIGURE 2.

Referring to FIGURE l, a carbonaceous stratum 10 is penetrated by anignition borehole 12 and surrounded by a number of air injectionboreholes 14, two of which are shown. Each of the boreholes is providedwith casing 16 and air injection tubing 18 is positioned in boreholes14. A production tubing string 20 comprising a perforate bottom secton22 is positioned in ignition borehole 12. Charcoal in the form ofbriquettes forms an annular packed bed 24 between the wall of theborehole and section 22. Perforations 26 in section 22 are smaller thanthe briquettes and provide a passageway into the tubing string so thatgases may pass between the stratum and the tubing thru the charcoal masswithout blowing the charcoal into the tubing string. If desired,perforations 26 may be in the form of elongated slots extendinglengthwise or circumferentially of the tubing as shown in FIGURE 2.Numeral 27 designates a fire front established in stratum 10 aroundborehole 12, moving outwardly toward boreholes 14. An air supply line 28connects with casing 16 of borehole 12.

Operation utilizing the arrangement shown in FIGURE 1 comprises ignitingthe mass of charcoal'24 by soaking the charcoal with a highly flammablehydrocarbon fluid, such as gasoline, injected thru tubing 20 (or priorto the positioning of the charcoal in the borehole) and igniting the uidby a spark, a squibb, or other suitable means so that the entire mass ofcharcoal is ignited with the aid of air injected thru the stratum fromborehole 14. Air may be injected thru line 28 to effect the ignition orassist in the ignition of the charcoal. The combustion gas from theignition is exhausted thru line 20. As combustion of the charcoalcontinues, the temperature of stratum 10 adjacent borehole 12 is raiseduntil the ignition point is reached at which time the in-placecarbonaceous material is ignited in the presence of free oxygen passingthru the stratum to the ignition borehole.

Since the temperature of burning charcoal under the conditionsmaintained in the bed of charcoal reaches the level of 2500 F. andhigher, reduction in the permeability of the surrounding stratum wouldresult with direct injection of air which carries the heat into thestratum; however, with inverse air injection the passage of air into theignition borehole prevents excessive heating of the stratum and therebyavoids damage and loss of permeability of the surrounding stratum. j

After a combustion zone has been established within the stratum adjacentborehole 12, continued injection of air thru the injection boreholesmoves the combustion zione Vvinjection boreholes. When the combustionzone arrives at the injection boreholes these boreholes may either beclosed or utilized as production boreholes with injection thru an outerring of boreholes being commenced upon termination of the injection thruboreholes 14 so as to move the combustion zione outwardly into unburnedstratum. The arrangement shown in FIGURE l also illustrates a boreholearrangement wherein initiation of ignition around each of a series ofignition boreholes 12 in a line of boreholes in a stratum flanked oneach side by a line of injection boreholes 14, is effected, in whichcase the operation is similar to that described with a ring typearrangement of injection boreholes around a central ignition borehole.

When utilizing a packed ponous mass of fuel, such as charcoal, inborehole 12 without axial tubing section 22,

the charcoal is ignited from the top by dropping a mass vof burningcharcoal onto the charcoal bed or by other suitable means and theinjection of air thru the stratum into borehole 12 effects burning ofthe charcoal downwardly from the top, thereby establishing ignition instratum 10 around borehole 12 progressively from the top to the bottomthereof. In s uchan operation tubing string 29 may be utilized toWithdraw combustion gases, picking up these gases at a level just abovethe 'wherein the charcoal fuel supply -is formed in a gas-pervious massin the form of a cylinder 30 bonded to a perforate tubing section 22.The perforations in tubing section r22 are in the form of longitudinalslots 32, providing a 'passageway for gases into the tubing or from thetubing into the charcoal. YThe charcoal cylinder is of slightly lsmallerdiameter than borehole 12 and casing 16 to facilitate positioning of thefuel element in the borehole and to provide a narrow annulus between thecharcoal and the lwall of the borehole. The lower end of tubing section22 'is preferably closed so that gas passing from the stratum into thetubing must pass thru the charcoal.

lThe charcoal fuel element in FIGURE 2 is of a length corresponding tothe thickness of stratum 10 and in this form it is ignited along itsentire extent in any suitable manner as hereinafter described andthereby simultaneously ignites the stratum over its entire cross sectionas air 4is fed thru the stratum into the borehole.

' In another embodiment of the invention utilizing a preformed fuelelement such as 30 on the end of a rod or tubing string, the fuelelement is prepared in a form substantially longer than the thickness ofthe stratum to be ignited and only the lower end of the fuel element isinitially ignited. Feeding air thru the stratum into the ignitionborehole then causes only the lower end of the fuel element to burnthereby producing maximum heat in the stratum adjacent the burningsection of the fuel element. The burning end of the fuel element may bepositioned at the bottom of the stratum whereby the element isprogressively burned upwardly from lthe lower end and rod or tubing 20is lowered into the borehole as the burning proceeds and as fuel element30 becomes shorter. The rate of lowering the supporting tubing or rod isregulated so as to maintain the burning end of the fuel element at theupper edge of the combustion area in stratum 10 so that the stratum isprogressively ignited along the length of the borehole therein upwardlyfrom the bottom. In this type of operation, fuel element L30 may besolid without an axial passageway so that tubing section 22 does notinterfere with lowering the burning end of the fuel element into theborehole as fuel is consumed and the fuel element becomes shorter.Tubing string 20 may, however, extend into the ,fuel element asubstantial distance and bey open on the lower end toexhaust combustiongases from the burning fuel thru an axial passageway thru the fuelelement. It is also feasible to ignite the lower end of fuel element 3i)with the same opposite the upper surface of stratum l0 and progressivelylower the burning end thereof into the stratum as ignition in thestratum is effected and the fuel rod becomes shorter, thereby ignitingthe stratum progressively downwardly from the upper level thereof.

The process of the invention offers good control of the burning rate ofthe charcoal by controlling the back pressure on the ignition wellthereby regulating the supply of oxygen entering the ignition borehole.Another advantage lies in the fact that direct air injection thru theignition borehole passes hot gases nto the stratum thereby melting orrendering more fluid heavy viscous hydrocarbons in the stratum which aredriven deeper into the stratum and upon cooling congeal there and shutoff the supply of air by rendering the stratum impermeable. By utilizinga bonded mass of charcoal as the fuel, or by arranging the mass of fuelaround a perforate tubing section in the stratum, the blowing ofparticles of charcoal out thru the ignition well with combustion gasesand/ or air is completely avoided with `only the ashes from the charcoalbeing blown out thru the production tubing. The inverse airinjectiontechnique also permits simultaneous ignition of the entire pay stratumaround the ignition borehole which is less feasible with directinjection thru the charcoal into the pay zone. Y

'Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

We claim:

1. A process for igniting a combustible gas-permeable carbonaceousstratum comprising suspending a bonded unitary mass of solid fuelcomprising charcoal in an ignition borehole adjacent said stratum;heating and igniting a lower end section of said mass of fuel; passingOZ-containing combustion-supporting gas into an offset borehole and thrusaid stratum into said ignition borehole to burn the ignited mass offuel, so as to heat and ignite a section of said stratum adjacent theburning fuel; thereafter moving the burning mass of fuel along saidignition borehole while passing said gas thru said stratum so as toignite an adjacent section of said stratum; and venting gases from saidignition borehole.

2. The process of claim 1 wherein the burning section of said mass offuel is moved progressively up said ignition borehole from the bottom tothe to-p of said stratum so as to ignite the entire exposed section ofsaid stratum within said ignition borehole.

3. The process of claim l wherein the burning section of said mass offuel is moved progressively down said ignition borehole from the top tothe bottom of said stratum so as to ignite the entire exposed section ofsaid stratum within said ignition borehole.

4. The process of claim 1 wherein said fuel mass is supported on the endof a tubing string and combustion gases are removed thru said string.

5. The process of claim l wherein said stratum is ig- Dited adjacent thebottom and the ignited area is caused to move progressively upwardlytherefrom to a level adjacent its top.

6. A process for initiating in situ combustion in a permeablecarbonaceous stratum comprising placing a gas-permeable mass of charcoalin a borehole within said stratum; heating said charcoal mass toignition temperature; passing O2 containing, combustion-supporting gasthru said stratum from an injection area into said borehole and saidcharcoal so as to ignite and continue combustion of said charcoal,thereby heating the adjacent stratum; continuing the passing of said gasthru said stratum so as to continue said combustion and to ignite thecarbonaceous material in said stratum after said stratum reachesignition temperature to establish in situ combustion therein; andventing from said borehole all gases produced by the combustion so as toavoid passing hot gases into said stratum and plugging thereof withheavy hydrocarbon material.

7. The process of claim 6 wherein a mass of charcoal briquettes ispacked into said borehole.

8. The process of claim 7 wherein the top of said mass of briquettes isadjacent the top of said stratum and said mass is ignited at the topwhereby combustion proceeds downwardly thru said mass the thickness ofsaid stratum so as to ignite the entire wall of said borehole withinsaid stratum.

9. The process of claim 6 wherein said charcoal is soaked with a liquid,water-repellant, combustible material prior to placement in said stratumso as to minimize absorption of formation water.

` 10. A process for initiating in situ combustion in a premeablecarbonaceous stratum comprising positioning a gas permeable mass offuel, provided with an axial gas passage therethru, in a borehole withinsaid stratum; igniting said mass o-f fuel; burning said mass of fuel byinjecting combustion-supporting gas into said stratum at a locus spacedfrom said borehole so as to pass said gas into said borehole and intosaid mass of fuel, thereby heating and igniting the carbonaceousmaterial in said Stratum adjacent said borehole; passing combustion gasfrom the burning fuel and said material into said axial passageway,upwardly therethru, and out of said well.

11. The process of claim l0 wherein said fuel mass is bonded together ona perforate section of tubing prior to positioning within said stratumto form said fuel and said tubing connects with a tubing leading out ofsaid borehole.

References Cited in the le of this patent UNITED STATES PATENTS

1. A PROCESS FOR IGNITING A COMBUSTIBLE GAS-PERMEABLE CARBONACEOUSSTRATUM COMPRISING SUSPENDING A BONDED UNITARY MASS SOF SOLID FUSELCOMPRISING CHARCOAL IN AN IGNITION BOREHOLE ADJACENT SAID STRATUM,HEATING AND IGNITING A LOWER END SECTION OF SAID MASS OF FUEL, PASSINGO2-CONTAINING COMBUSTION-SUPPORTING GAS INTO AN OFFSET BOREHOLE AND THURSAID STRATUM INTO SAID IGNITION BOREHOLE TO BURN